Ok, I have a older system I built years ago, but it still runs good so I havent upgraded yet.
Recently I started playing some computer games again (all low game settings) and I went from getting 125fps to as low as 40fps, and it was very laggy and choppy (not an internet problem for sure). I would exit the game, then go back in about 10 minutes later, and sure enough 5-10 min into the game, it started getting really choppy. I opened the case, made sure the fan was running, and touched the top of the card where the chipset would be on the other side, and it was very hot. I couldnt keep my finger there long.
I took the card out (which is a ATI Radeon 9800 Pro), and took off the heatsink and fan. I cleaned up the chip, cleaned the heatsink, and put some arctic silver on. Everything put back together, I tried running the game again. No problems for two days, and I even played enough for it to get hot. Today, I played again. It went back to the same problem as before. I opened the case back up, and the surface on the other side of the chipset was very hot.
I ran antivirus, and no issues. BTW, the game I was playing was the same game that Ive played on and off for years (with the same setup) and I have never ever had an issue.
No I dont know how to check a graphics card temp without a external thermometer which I dont have. All I know it that it was hot enough to not keep my finger there.
Im wondering why there would be a heat issue when I basically cleaned it back to brand new? I would get it if I changed the heatsink or something but I didnt. Unless graphics cards can just overhead if they get old I dont know.
There are test programs out there to test temps like GPUZ and CPUZ. I would start with something like memtest86 and see if you are getting any failures. Then do prime95 and set it on small FFT's to see how the processor temperature reacts. Then after that passes go to large FFT's to see how the whole system reacts. This should take out your CPU/GPU/Memory as being the issue. You still could have an north/southbridge heat issue which you could look into also.